The real world moves. If your code didn't change it must be generating value against something that is very standard but I'd be very surprised that you're not modifying, adding, etc based on usage to derive even more value somehow.

> Code that doesn't need to change is a really good sign that you've got something good.

Not really, but you're thinking in terms of "my code nailed it because I'm not touching it" and that has NOTHING to do with it being easy to change.